I will explain as following how extra price work:
Q: What’s “Extra price” ?
A: It mean that you will have to pay more for extra price if the people of party more than the people of Base Price.
Example in this case:
When you book 2 people, here just only 1 addition people not 2 because you entered the Base price for 1 people, so that’s reason why afer check availability your price don’t change.